The present invention relates to an intake duct such as an intake manifold connected to suction ports of a cylinder head.
Japanese Laid-Open Patent Publication No. 2012-102623 discloses an intake duct that includes a main passage, an auxiliary passage, a partition separating the main passage and the auxiliary passage from each other, and a valve for opening and closing the main passage. When the engine combustion tends to be unstable, such as during a cold start, the valve closes the main passage so that intake air flows through the auxiliary passage. This increases the flow velocity of the intake air, generating flows such as tumble flows of the intake air introduced into the cylinders. As a result, the engine combustion is stabilized without significantly increasing the fuel injection amount.
In the intake manifold of the above document, the cross-sectional area of the auxiliary passage decreases toward the downstream side and is minimized at the downstream end. This further increases the flow velocity when the intake air flows through the auxiliary passage, intensifying the flows such as tumble flows, which are introduced into the cylinders. The engine combustion is further stabilized, accordingly.
The passage cross-sectional area at the downstream end of an intake duct (the sum of the cross-sectional area of the main passage and the cross-sectional area of the auxiliary passage) is determined with reference to the cross-sectional area of the intake port to which the downstream end is connected. Therefore, in a structure in which the cross-sectional area of the auxiliary passage is minimized at the downstream end and increased toward the upstream side as in the intake duct of the above document, the following problems may be present. That is, since the cross-sectional area of the auxiliary passage is increased at the upstream end, the cross-sectional area in the vicinity of the upstream end may be greater than the ideal cross-sectional area for maximizing the intake efficiency of intake air into the cylinders in an engine operating state with the valve open. This results in an insufficient intake of air into the cylinders, which restrains the engine power from being increased.